The present invention relates to a device for treatment of wastewater. More particularly, the present invention relates to a device for the removal of biodegradable contaminants from wastewater using biological processes. The present invention also relates to a system for purifying highly contaminated water which contains a large amount of suspended solid impurities (TSS) and high concentrations of BOD (biochemical oxygen demand) and COD (chemical oxygen demand) as, for example, effluents from food processing plants and toilets. The device enables the efficient removal of biodegradable solid substances from wastewater by a combination of filtration and biochemical reaction. In other words, this invention relates to a bioreactor device that enables both biological degradation and the filtration of suspended solids.
The treatment of wastewater for the removal of substances that cause BOD is best accomplished by biological treatment methods such as aerobic degradation and anaerobic degradation. There is a wide range of industrial operations that result in liquid effluents containing substantial amounts PBOD, which has to be removed to conform to national discharge regulations. In addition, several of these high-BOD industrial effluents, have substantial quantities of BOD present as suspended solid material.
In general, examples of BOD exerting suspended solids include, but are not limited to, partially dissolved and partially macromolecular materials, such as proteins, long-chain fatty acids, fats, vegetable oils, tallow, bacterial and yeast cell-walls, celluloses, hemicelluloses, and starch; the suspended solids being present in emulsified, suspended or colloidal state. Effluents of this nature are discharged for example, from slaughterhouses, dairies, rendering plants, oil mills, pharmaceutical and organochemical plants, pulp and paper factories.
The relevant organic compounds constituting biodegradable solid organic matter are generally classified as 1) Polysaccharides common among which are cellulose, hemicellulose, starch and pectin 2) Proteins and amino acids which are present as insoluble matter when coagulated by heat, acids or tannins and 3) Fats and long chain fatty acids. All these compounds can be degraded by anaerobic micro-organisms to form methane. The first step in the degradation process is called solubilization which results in the formation of soluble compounds and is carried out by enzyme action outside of the microbial cell. This is a slow process and requires sufficient microbial cells producing the enzymes and sufficient contacting time between enzyme and solids. In fact, the solubilization step is most often the rate limiting step in the sequence of anaerobic reactions that result in mineralisation of the polluting substances.
The development of high-rate anaerobic reactors, anaerobic treatment has become the economic option in the pretreatment of high BOD industrial effluents. However, high-rate reactors can be used only for the treatment of industrial effluent with BOD in primarily dissolved form. There are no high-rate reactor devices, in use, for the treatment of complex wastewater, i.e., wastewater containing un-dissolved BOD. A survey of prior art, has revealed no apparatus specifically designed to accomplish the primary object of this invention, i.e., the high-rate anaerobic treatment of wastewater containing suspended solids. Therefore, a survey of related art is given below, wherein, some ideas and concepts related to this invention may be found. This survey is merely in support of the practicality of the concepts used in this invention and does not in any way detract from the absolute novelty of the device.
Related Art in Anaerobic Treatment of Wastewater:
High-rate anaerobic reactors enable continuous treatment of industrial effluents at small hydraulic retention times. In other words, reactor sizes are relatively small and the BOD load per unit reactor volume per day is high. The primary principle that permits high BOD loading is the retention of a large population of viable microorganisms (biomass) within the reactor by decoupling the retention time of microorganisms from the hydraulic retention time. In simple language, the microbes stay within reactor longer than the liquid being treated.
A variety of high-rate anaerobic reactor systems are in commercial use. A good description of anaerobic processes and reactors is given by S. Stronach, T. Rudd and J. Lester, xe2x80x9cAnaerobic Digestion Processes in Industrial Wastewater Treatmentxe2x80x9d, 1986, Springer Verlag, Berlin. The prominent designs can be classified into three families: 1) fixed-film reactors 2) sludge-bed reactors and 3) fluidized bed reactors. However, none of these prior art high-rate anaerobic reactor systems are suitable for the treatment of wastewaters where a substantial quantity of BOD is present as solid matter. This will be clear from closer examination of the operating principles and constructional features of these reactors.
The fixed-film reactors use a stationary inert packing media within the reactor on which microorganisms are retained as a biofilm. The function of the media is enhanced by the increasing the surface area. But the media should have sufficient open porous channels available for free flow with only minor hydraulic head loss and these channel should remain open even after copious growth of biofilm. Among various media types available, are dumped or random packing comprising rings like units and structured media constructed with corrugated plastic sheets jointed to form blocks with dividing straight channels. Both types of packing media are rapidly choked by the deposition of suspended solids present in wastewater. This is especially true in the case of fully submerged upflow type fixed film reactors. In a downflow configuration, the presence of solids in the wastewater will either lead to choking of media or if media has sufficient porosity and straight channels for free outflow of solids, there will be insufficient removal of BOD.
The sludge-bed reactors enable the retention by using the settling property of sludge which is denser than wastewater, if free of gas bubble inclusions. Specially designed xe2x80x9cgas solids separatorsxe2x80x9d are mounted on top of reactor to enable settling of sludge. While these reactors are affected by choking problems as is the case with fixed film reactor, they are still not suitable for the treatment wastewater containing solids. Solids in wastewater will rapidly attach or adsorb to the sludge, decreasing its specific gravity and impairing the settleablity of sludge. This lead to the phenomenon of sludge washout, a common and recurring cause of failure of several installations of such reactor. In general, sludge bed reactors are not recommended for wastewater with more than 30% of COD present as insoluble matter. The originator of UASB (upflow anaerobic sludge bed) technology, Professor Gatze Lettinga, from the Netherlands writes in his comprehensive review: xe2x80x9cUASB process design for various types of waste-waterxe2x80x9d; Water Science and Technology, 24, 8, 87-107 (1991), xe2x80x9cRegarding the reactor system to be chosen for treating a partially soluble complex wastewater it will be clear that process that apply a high superficial velocity, such as fluidized bed and expanded granular sludge bed (BOSS) reactors are unsuitable, unless they are combined with an adequate pre- or post-clarifier. According to our present experience, application of granular sludge UASB (or EGSB) reactors becomes doubtful at TSS-concentration in the influent exceeding 6 to 8 g/l because at such high TSS-concentration, the segregation between granular and flocculent sludge does not proceed sufficiently rapidly. For high strength wastewaters with high insoluble fraction, (i.e., exceeding 15%) generally conventional digesters are in favour over UASB and other high-rate systems.xe2x80x9d
Fluidized bed reactor using small biofilm carrier particle in fluidized state is free of clogging, choking or sludge washout problems. But fluidized bed reactors are still not suited for treatment of wastewater with insoluble BOD as solids are not retained in the reactor for sufficient time for the solubilization.
The only reactor system which is suitable for the anaerobic treatment of solid containing industrial effluents is the stirred tank digester, also known as sludge digester. This reactor system is a low rate reactor system where no attempt is made to delink liquid residence time from suspended solid residence time nor is there any attempt to retain and increase the microbial biomass in the reactor. The stirred tank digester essentially maintains a homogenous mixture of solids and liquid for sufficient long duration to effect degradation.
There is only a limited attempt in prior art to develop a high rate reactor specifically for the anaerobic treatment of solids containing effluents. In order to overcome the inadequacies of anaerobic filters and encapsulated bacterial retention systems in treating organic materials with suspended solids content, the prior art has identified the use of the so-called anaerobic activated sludge process, which has also been called the anaerobic contact process, see S. Stronach, T. Rudd and J. Lester; xe2x80x9cAnaerobic Digestion Processes in Industrial Wastewater Treatmentxe2x80x9d, 1986, Springer, Verlag, pp. 93-120, 136-147. The anaerobic contact process uses a secondary solids separation stage downstream of the anaerobic reactor and the separated solids are recycled back to the reactor. The secondary separation stages may be gravity settling or mechanically separation such as centrifugation or flotation.
U.S. Pat. No. 5,015,383 describes the difficulties with the anaerobic activated sludge as follows: xe2x80x9cThe anaerobic activated sludge, or anaerobic contact process, has not been effectively utilized because the bacteria in anaerobic digestion are not easily separated from the mixed liquor effluent. The difficulty his been that actively fermenting organisms do not settle by gravity because of the buoying effects of attached gas bubbles and the fact that the density of the bacteria closely approximate the density of water and do not floc easily. The use of other common liquid/solids separators also have disadvantages. The use of gravity clarification with the addition of high concentrations of flocculating or coagulating chemicals is expensive and harmful to the bacteria. Rapid temperature and pH changes have also been attempted and found to be harmful to the bacteria. Centrifuging has been found to be expensive and detrimental to the bacteria. Conventional dissolved air flotation as well as froth and foam flotation techniques are detrimental to the anaerobic bacteria since even minute amounts of oxygen or air are sufficient to destroy the bacteria. S. Stronach, T. Rudd and J. Lester, Anaerobic Digestion Processes in Industrial Wastewater Treatment, 1986, Springer, Verlag, pp. 35-38xe2x80x9d. The Burke patent teaches a method of separation using gas flotation, in particular the use of biogas as flotation gas, to overcome separation problems. It would be clear, that this process is hampered by the expense and complexity of the gas flotation process involving mechanical solids removal systems and carried out in closed equipment with a flammable gas. The effectiveness of this process is not known and there is no known installation of the process.
U.S. Pat. No. 4,551,250 teaches a process by which wastewaters containing undissolved solids are treated in a two reactor system in series. A filtering process retains solids in the first reactor of sufficient residence time to be degraded to low molecular weight soluble forms which are passed. While the basic concept of retaining solids for sufficient time by decoupling suspended solids residence time from the hydraulic residence time is the same as in the present invention, there is no mention in U.S. Pat. No. 4,551,250 of the certain possibility of clogging of filter media nor about methods by which this prevented. In contrast to my invention, the process disclosed specifically is for a first reactor to be operated at a low pH in order to inhibit gas production. In a further point of deviation from my invention, the first reactor in U.S. Pat. No. 4,51,250 is preferably maintained in a quiescent state to avoid turbulence and enhance settling. In a still further deviation, the filtering process is accomplished by biofilm media provided in the first reactor. Now, it would be clear to those in the art, that solids settling and clogging of biofilm media is a major problem in such reactors when treating high strength wastes, and in particular wastes with suspended solids. The declogging of such systems is an almost impractical task. In any case, U.S. Pat. No. 4,551,250 does not teach any apparatus that would specifically enable the operation of the process while overcoming the obvious problems with respect to filtration of heavily contaminated wastewaters.
A rather crude apparatus and method for the treatment of complex wastewaters is disclosed in British Patent 2,167,055. This system comprises an anaerobic followed by a downflow or upflow filter. No method of backwashing or declogging of the filter bed is mentioned. It is obvious that this does not support high-rate operation and may be considered merely a modification of a pond treatment system, and may be applied only where a pond is considered appropriate.
A reactor system which theoretically can function with solids containing effluents are membrane bio-reactor systems wherein membrane modules are used for separation of solids from the liquor. These systems have not been successfully applied for anaerobic processes because of membrane fouling problems, which are more than in aerobic processes. In addition, these reactor systems consume large amounts of energy to drive the membrane systems. It may be noted that membrane systems separate very fine and colloidal matter including individual microorganisms in addition to larger suspended solids present in the effluents of interest. Thus the application of membrane systems for the anaerobic treatment of complex wastewaters is an overkill being inappropriate and uneconomic for the application at hand.
Reference may be made to U.S. Pat. No. 4,613,434 which teaches a device for treatment of wastewater by means of anaerobic fermentation comprising a single reactor within which is a lamella and biofilm filter divides the reactor into a mixed lower zone and an upper zone. Although, the lower zone is claimed for xe2x80x98acidificationxe2x80x99 and the upper zone is claimed for methanogenesis and there is no mention anywhere in the patent of solids hydrolysis or the filtering of solids, let us assume that the apparatus is applied for suspended solids containing wastewater. It will be immediately clear that the apparatus has no mechanism for clearing choking of filter as a result of retention of filtered solids. The lamella filter in lower zone is of no use for buoyant solids which include fats that are usually lighter than water and other solids with occluded gas bubbles which those in the art will recognize immediately.
Reference may be made to U.S. Pat. No. 5,314,621 disclosing a method for biological purification of wastewater by upflow of wastewater through a buoyant biological filter bed of expanded polystyrene beads with simultaneous injection of air flowing co-currently with through the bed. The filter is backwashed as per the invention by backflushing at a rate of 30 to 80 m/h with treated water stored at an upper part of the reactor. What is non-obvious as per the patent is the provision of brief mini-backflushing operations that expands the filter bed just sufficient to loosen suspended solids within the bed and enable deeper penetration of impurities into the filter bed, thus enabling a longer filter run. It may be noted that the invention is not for anaerobic treatment processes and, further, provides no specific apparatus.
A study undertaken by Nuri Azbar, Pepi Ursillo and Richard E. Speece found in Water Research 35, 3, 817-829,2001 of the effect of reactor configuration and substrate complexity on the performance of anaerobic process is relevant to the present invention. Based on their experiments with various types of well known reactor, they write: xe2x80x9cThere appears to be something profoundly beneficial to phasing and staging of anaerobic treatment process for the substrates studiedxe2x80x9d which include simple molecules such as volatile fatty acids and complex mixed substrates such as baby formula.
There is no known teaching in prior of any special apparatus for the anaerobic treatment of solids containing effluents. Prior art only reveals processes and methods of handling such effluents using known apparatus.
Related art in deep-bed filtration, a common water treatment method for removal of suspended impurities:
Buoyant bed filters form a variety of devices in the well known class of granular bed filters prior art. See for example. xe2x80x9cBuoyant media filterxe2x80x9d in U.S. Pat. No. 4,446,027 issued May 1, 1984 which uses a 12 inch deep filter bed constructed from commercially available hollow glass beads 0.7 mm in diameter to achieve very good suspended solids removal performance. These devices have so far been intended solely for turbidity removal in water treatment and apparatus is provided for cleaning of the clogged filter by back washing or back flushing. None of the prior art devices and backwash methods are suited for or have the advantage of removal of high concentration of biodegradable suspended solids in heavily polluted industrial waters, which requires the combination of bioreactor and filter.
Another related art is found in xe2x80x9cbead filterxe2x80x9d devices disclosed for use in nitrification and filtration of aquaculture water. Inventor Robert Malone has shown such devices U.S. Pat. Nos. 5,445,740; 5,126,042; and 5,232,586 wherein a floating filter is used as biofilm carrier device and a filter device for accomplishing nitrification of aquaculture wastewater. In particular the device shown in U.S. Pat. No. 5,232,586 employs a tank having an upper filtration chamber and a lower expansion chamber fluidically connected to each other by a constricted passageway. An inlet line supplies water to the tank through the lower chamber, while a floating media pack forms within the upper chamber during filtration, an outlet line is connected to the tank above the media pack and delivers filtered water back to the aquatic environment. Back-washing is accomplished by the displacement and expansion of the media pack through the constricted passageway using raw water directed to the upper chamber. The turbulence of this expansion causes the filtered matter and sludge to fall toward a drain line located at the bottom of the tank. Since no aeration provision is provided, the apparatus is suitable only for nitrification using dissolved oxygen already present in the wastewater, i.e., only for low concentration ammonia removal. This apparatus is both unintended and unsuitable for the anaerobic treatment of high suspended solids wastewater, as there is no separate provision for gas collection, and gas bubbles will quickly accumulate in the floating media, it is also clear that the apparatus is intended for filtration of wastewater with suspended solids concentration an order of magnitude lower than that of the present invention. U.S. Pat. No. 5,232,586 invention is merely mentioned as a related art and the simple device therein bears little comparison to our reactor designed to meet the myriad operational requirements that a anaerobic reactor high strength complex wastewater. It is therefore clear that these apparatuses do not in any way impinge on the novelty of our invention not only by way of mechanical arrangement of apparatus, but also by way of concept.
Some related art in sewage treatment is given herein as our present invention, in addition to its utility in industrial wastewater treatment, also provides a compact and efficient apparatus for treatment of domestic sewage. Sewage is low-strength wastewater and contains BOD in mostly undissolved form. Sewage treatment can take two formsxe2x80x94centralized treatment plants in towns served by sewerage systems and decentralized home scale or colony scale units where no-frill sewerage systems are not available.
The following quote from xe2x80x98A review: The anaerobic treatment of sewage in UASB and EGSB reactorsxe2x80x99; see Lucas Seghezzo et al., Bioresource Technology 65,175-190 (1998), reveals the importance of a high-rate anaerobic reactor systems for sewage. According to Jewell (1985), xe2x80x98there is little doubt that development of cost-effective and efficient anaerobic sewage treatment alternative would be one of the most significant advances in waste treatment historyxe2x80x99. Lettinga et al. (1987) fully agreed with this statement by saying that . . . a satisfactory application to raw domestic sewage would represent the maximum possible accomplishment for high-rate anaerobic treatment systems"". The term xe2x80x98high-rate was once used for the later design of sewage sludge digesters, but it now widely used to refer to anaerobic treatment systems meeting at least the following two conditions: (a) high retention of viable sludge under highloading conditions, and (b) proper contact between incoming wastewater and retained sludge (Lettinga et al., 1987)xe2x80x9d
Anaerobic treatment has been applied with moderate success for the centralised treatment of sewage in hot climates. UASB type technology is used for primary treatment of sewage. The sewage COD loading rate for UASB type reactors is low (less than 1.5 kg/m3/d) as the primary mechanism of removal of COD and BOD is entrapment of suspended solids in the anaerobic sludge of the UASB reactor, and subsequent degradation. In this manner, suspended solids residence time is decoupled from the hydraulic residence time. But, higher loading rates will lead to sludge washout and failure of the system. Our invention being able to retain solids by active filtration can achieve higher COD and BOD loading without danger of sludge washout. Further it would be clear from the description of the apparatus that floating matter is also effectively retained in our apparatus to complete the degradation process.
In the case of decentralized sewage treatment, septic tanks are effective and commonly used the world over. A septic tank is basically an anaerobic reactor with baffle arrangement to provide sufficient solids detention time to effect degradation. Septic tanks can be used for treatment of composite domestic sewage, or for the treatment of black water from toilets only. The residence time for composite sewage in septic tanks is at least 24 hours, BOD loading rate of the order of 0.3 kg/(m3d). Septic tanks are designed to provide sufficient sedimentation time for separation of solids from sewage and sufficient volume to provide solids residence time for degradation. Higher loading rates are possible in UASB systems because of better contacting and mixing than in septic tanks, enabling lowering of residence time to 6 hours. At even lower residence time, sludge washout occurs in UASB systems. The effective retention of solids by filtration by our buoyant filter bioreactor, and better contacting and mixing conditions enable the anaerobic treatment of sewage in a compact apparatus. No such filter reactors are available in prior art although several filters for post treatment removal of solids and pretreatment removal of solids from septic tanks are mentioned. The present invention is conceptually different from any of prior art devices septic tank and filter systems, being essentially different in principle, configuration, and method of working, by its use of buoyant filter and gas driven backflush system. Merely for the sake the completeness, a brief review of some septic tank filter devices is given below.
Sewage disposal apparatus employing circulating filter media is shown in U.S. Pat. No. 5,308,479. A filter bed using buoyant media is provided for filtration and the media is prevented from clogging by use of a circulating fluid flow generated by either a propeller type agitator or by air sparging. The method in accordance with U.S. Pat. No. 5,308,479 attempts to prevent the clogging of the filter media by moving it continuously in a slow circulation with media particulates being maintained at substantially fixed relative positions during operation. There is no attempt to perform a normal filtration operation (obviously a more effective filtration operation as compared with a moving bed), and intermittent back flush of media by fluidization which are essential in our invention. The U.S. Pat. No. 5,308,479 invention is suited for totally unpowered gravity flow operation unlike our invention.
Suspended solids-free effluent from septic tanks is desired also to prevent clogging of drain field to which it is discharged. Some invention of septic tank filters have this objectivexe2x80x94see for example: simple basket type filters U.S. Pat. Nos. 5,198,113 and 6,177,004 or simple pipeline filter U.S. Pat. No. 6,136,190, to a complex and rather impractical layered filter as taught in U.S. Pat. No. 6,024,870. These inventions do not seek to enhance the degradation capacity of the septic tank by improving the loading rate.
A primary object of the present invention is to provide a reactor system suitable for anaerobic treatment of complex wastewater containing substantial amount of BOD as insoluble matter.
Another object of the present invention is to provide a device that enables the solubilization of biodegradable solids and the further conversion of produced soluble compounds into biogas, the two steps being carried out sequentially in separate compartments arranged in a compact unit.
Another object of the present invention is to provide an apparatus which is capable of effectively removing BOD and suspended matter from the water without requiring a long residence time or increasing the size of the processing vessel.
Still another object of the present invention is to provide an apparatus which enable recovery of biogas.
Yet another object of the present invention is to provide a filter system which is backwashed and de-clogged automatically without the actuation of mechanical or electrical valves or devices with moving parts.
A further object of the present invention is to provide a filter reactor system which is backwashed and declogged whenever the filter pressure drop exceeds a set value.
Yet another object of the present invention is to provide a filter reactor system which is backwashed and declogged at a regular interval.
Still another object of the present invention is to provide a reactor with a filter system that is automatically declogged whenever the filter pressure drop exceeds a predetermined value.
Still another object of the present invention is to provide a reactor that removes biodegradable, settleable and filterable matter from wastewater
Still another object of the present invention is provide an anaerobic reactor, wherein anaerobic bacterial sludge is retained within the reactor even when it has a floating tendency as a result of adsorption of low density material such as fats.
Still another object of the present invention is to provide a high rate anaerobic reactor which does not require external energy input for mixing and agitation.
Still another object of the invention is to provide a automatically declogged floating media filter reactor which has a minimum of moving parts and is easy to operate and maintain.
Still another object of the invention is to provide a filter bioreactor that does not require periodic stoppage of flow for backwashing.
Still another object of the invention is to provide a filter reactor with a gas driven backflushing system, which may, where required, operate only on self produced gas.
Yet another object of the present invention is to provide a multistage anaerobic reactor which can remove successively finer suspended particles and breakdown products.
Yet another object of the present invention is to provide multistage anaerobic reactor which can provide a high efficiency for the removal of both suspended and dissolved organic contaminants in a single and compact reactor.
Yet another object of the present invention is to provide a high rate anaerobic reactor, which can be made tall and slender so as to occupy a small footprint.
Still another object of the present invention is to provide a septic tank and aerobic treatment system that can be used for the treatment of black water sewage from single or cluster of dwellings.
Still another object of the present invention is to provide a treatment device that can carry out anaerobic treatment and aerobic treatment of black water sewage without external power sources and without loss of hydraulic head.
All the above objects are achieved by the invention described herein as the buoyant filter bed reactor with gas-driven backflushing and its various embodiments.
The present invention is the first high-rate reactor, (applying a high superficial velocity), that is capable of treating partially soluble, complex, high-strength wastewaters. Our invention is the first single reactor system in a compact and mechanically simple package that is configured in multiple stages and has the ability to retain complex insoluble substrates in a wide range of particle sizes in spatially separate stages for sufficient residence time to enable complete degradation. Therefore, a highly efficient performance for the removal of both solid and dissolved contaminants may be expected with the present invention. The present invention may be seen as a the first ever synthesis of a innovative self-cleaning deep-bed granular filter with staged anaerobic reactor to achieve the object of solids liquefaction by decoupling suspended solids retention time from liquid residence time. The present invention specifically provides a system for periodic declogging of the filter bed and retention of solids in a first stage digester for liquefaction. Our invention can also perform as a self-pumping system, which can discharge sewage at a hydraulic energy grade above that of the inlet sewage. This function is applicable in the case of high-strength blackwater sewage with COD exceeding 3000 mg/1. There is absolutely no mention in prior-art of a self-pumping system for any sewage or any other anaerobic reactor system.
Accordingly the present invention provides a device for the biological treatment of wastewater containing biodegradable solids comprising a vertically oriented elongated vessel partitioned, in vertical progression, by impermeable substantially horizontal walls, into a top chamber, a bottom chamber and, where required, a multiplicity of intermediate chambers one below the other, in between the top chamber and bottom chamber, each chamber having a gas retaining space and a liquid retaining space, wherein for any neighboring pair of chambers, the lower one is termed xe2x80x9clower chamberxe2x80x9d and the upper one is termed xe2x80x9cupper chamberxe2x80x9d, a nozzle establishing fluid communication between the bottom chamber and the outside of the vessel for input of wastewater into the vessel, a nozzle establishing fluid communication between the liquid retaining region of the top chamber with the outside of the vessel for discharge of treated wastewater from the vessel, and arranged so as to retain a level of liquid within the top chamber, a nozzle for discharge of gas from the gas retaining space of the top chamber, and further comprising for every pair of neighboring chambers, at least one filter chamber having at least one inlet communicating fluidly between liquid retaining part in the lower chamber and at least one outlet communicating fluidly with the liquid retaining part of the upper chamber, a filter bed constructed using a particulate bed of inert material with specific gravity less than 1.0, placed within the said filter chambers, and partly filling its internal volume, at least one gas conduit establishing fluid communication between gas retaining space in the lower chamber with the upper chamber, and a device enabling periodic discharge and stoppage of flow of gas through the said gas conduits.
In a preferred embodiment of the invention the reactor is divided by a fluid tight horizontal partition plate into upper and lower chambers. The filter chamber is constructed as modular units, using a length of pipe of suitable diameter, vertically penetrating through the partition plate. The module has perforations below a predetermined level in the cylindrical wall opening out into the lower chamber and an impermeable end cap at the bottom. At the top end, a perforated end cap is fitted, opening out into the upper chamber. The module is filled with the buoyant filter media, particle size chosen to effect required degree of filtration of the suspended solids in wastewater. The perforations in the module are chosen to be less than the diameter of the filter particles thus effectively confining the filter bed inside the module. As the bottom end cap is impermeable, it serves to deflect and prevent the escape of rising bubbles into the filter module. The apparatus operates by collection of gas above the liquid surface in the lower chamber below the horizontal partition in the space formed by the inner walls of the vessel, bottom surface of the horizontal partition and outer walls of the filter chamber, above the level of perforation in the filter chamber. The accumulation of gas drives the liquid in the lower chamber through the filter into the upper chamber. Suspended solids are filtered by the granular bed. When sufficient gas has collected in the lower chamber, it is released to the upper chamber.
In yet another preferred embodiment of the apparatus, the gas conduit can be shaped to form simple hydraulic automatic discharge and stop system. In this system, the gas conduit forms the shape of a U inside the lower chamber, one arm of the U opening out into the lower chamber and the other arm extending into the gas space of the upper chamber. The diameter of the conduit is chosen such that two phase flow of liquid and gas in the operational ranges of gas discharge is the regime of xe2x80x9cliquid plugxe2x80x9d pushed by gas and not in the bubble flow regime. This may be readily determined by those knowledgeable about two phase pipe flow. Initially, the short arm of the U is totally immersed in liquid in the lower chamber. As gas accumulates in the lower chamber the short arm of the U shaped gas conduit is exposed and a plug of liquid in the short arm of the U is pushed by the hydrostatic pressure into the rising longer arm, till it is entirely displaced from the shorter arm. At this stage, the gas pressure in the lower chamber gas space just exceeds the maximum hydrostatic pressure of the liquid plug which is then driven upwards and discharges into the gas space of the upper chamber. Thus, gas flow from lower chamber to upper chamber takes place, simultaneously with back flow of liquid from upper chamber to lower chamber through the filter bed. As the liquid level in the lower chamber rises and reaches above open end of the short arm of the U, the flow of gas is cut off by the formation of a liquid plug inside the gas conduit. The sequence of step is repeated as gas accumulation proceeds once again, thus enabling repeated and period backflushing. Further, in order to prevent the entry of suspended solids or floating matter into the U tube, a perforated hood device may be provided at its lower chamber entrance.
In a further preferred embodiment of the invention, gas recirculation to the lower chambers using a pump from a gas reservoir or from an upper chamber gas collection space is provided. This can be used to enhance the frequency of backflushing or obtain a longer duration backflush by supplementing gas generation The gas recirculation also aids in vigorous mixing of the reactor contents to improve mass transfer and enhance the rate of reaction.
In yet another embodiment, the apparatus of the invention may be operated as a sequencing batch reactorxe2x80x94xe2x80x9cfill, reactor and drawxe2x80x9d methodxe2x80x94in a most natural and automatic manner. In this embodiment, the wastewater to be treated is charged into the apparatus which already contains a volume of reacted liquor and a large population of anaerobic micro-organisms. The reaction commences producing gas in the lower chamber. The accumulation of gas in the lower chamber displaces the lower chamber liquor through the filter bed into the active sludge in the upper chamber, wherein further reaction and gas production occurs. At the end of the reaction, the reacted liquor may be withdrawn from the upper chamber.
Alternately, an overflow outlet may be provided in the upper chamber, so that treated effluent overflows from the upper chamber when the liquid level rises as a result of accumulation of gas in the lower chamber. Periodically the accumulated gas in the lower chamber is discharged, effecting the backflushing of the filter bed. The gas discharge cycle may be repeated several times if necessary before the reaction is complete. In this mode of operation, the volume of wastewater charged into the reactor during each filling cycle is equal to the volume of gas discharged during one gas discharged cycle and discharged of treated effluent takes place only during the first gas discharged cycle.
A further advantage of the invention is obtained if a non-return valve device is provided at the liquid inlet to the device, which does not permit backflow of liquid from the reactor. Initially the reactor contents are at a hydraulic level which enable the filling of a charge of wastewater by gravity flow. Upon filling, the non-return valve closes preventing reverse flow. As the gas generation proceeds, liquor is displaced to the upper chamber till it reaches an outlet provided at a hydraulic level which may be above the hydraulic level of the inlet stream. The reactor may be so configured so that the upper chamber is tall and narrow with respect to the lower chamber to enable pumping to higher elevation. In addition, the gas discharge volume may be chosen such that there is only one gas discharge operation per sequence. This enables the displacement of a large quantity of liquor from the lower chamber of reactor to the upper chamber. The advantage of this operation is that wastewater is discharged fairly continuously during the gas generation process, at a higher hydraulic level as compared to the source. Hence the reactor, besides providing anaerobic treatment, also provides the function of an equalization tank and pump system for downstream secondary treatment. It is conceivable that the pumping effect can be beneficially used for downstream treatment, for example, by an exposed aerobic trickling filter or a solar disinfecting basin, while the wastewater drain and suitable collection sump is underground. Therefore a totally unattended operation of the invention is possible with no external power source for anaerobic and aerobic treatment of highly contaminated wastewater.
Another embodiment of the invention is the multistage buoyant filter bed reactor, where there are several intermediate stages between the bottom stage and the top stage. The passage of liquid from bottom to top passes through filter bed at each intermediate stage. This enables very high efficiency of suspended solids and dissolved organic matter removal. The filter beds may be constructed so as to retain coarse suspended matter at the lower stage with each succeeding stage retailing progressively finer solids. This is easily accomplished by choice of the filter media and in particular the particle size of filter media. Gas collected in the bottom stage discharges periodically to the second stage, backflushing the filter bed connecting the two stages. As gas collection at the second stage exceeds the predetermined volume, it is discharged to the third stage backflushing the filter bed at the second stage. This sequence is repeated at each stage to set up a cascade till ultimately the gas is discharged from the system and all filters are flushed. Those in the art will acknowledge the extreme difficulty in filtration separation of wastewater containing large sized to fine sized suspended solids organic matter and will readily appreciate advantages of the multistage buoyant filter bed reactor of this invention which is not only able to separate but also to degrade the separated matter, in a single, compact and energy efficient device.